A superconducting nanowire avalanche photodetectors (SNAP) comprises two or more nanowires connected in parallel between an input contact and an output contact. In operation, the nanowires are cooled below their critical temperature so that they become superconducting. While cooled, a current runs through the nanowires between the input contact and the output contact. Because the nanowires are in parallel, the current is split equally among the nanowires. Illuminating one of the nanowires with a photon creates a hotspot with increased resistance, diverting current into the other nanowire(s). This causes the current in the other nanowire(s) to exceed the critical current, which is the current at which the nanowires transition from superconducting to resistive, and creates a voltage difference that can be detected across the entire device.
One advantage of a SNAP is the increase in the single-to-noise ratio (SNR) of the output of the detector by a factor of approximately the number of nanowires in parallel, N. A higher SNR allows thinner nanowires that are more sensitive to infrared (IR) photons to be used. However, the maximum SNR achieved to date remains below the theoretical value, even in SNAPs with meandering nanowires have been developed. This problem has also been observed in high fill-factor superconducting nanowire single photon detectors (SNSPDs) based on a single meandering nanowire.
The less-than-ideal SNR may be due to current crowding at sharp corners or non-optimal curves in the SNAP's meandering nanowires. Current crowding can lead to increased current density in the corner region compared to the current density in the rest of the device. Areas of current crowding switch to the normal state at device bias currents less than the switching current of the nanowires composing the rest of the device and thus limit the bias current at which the device can be operated.